Spinocerebellar ataxia type 6 (SCA6) is an untreatable neurodegenerative disease due to CAG repeat expansions in the gene, CACNA1A, that cause causing degeneration of cerebellar Purkinje cells and ataxia. There is no known treatment, although gene-silencing approaches targeted towards silencing the toxic protein have shown promise in other CAG repeat disorders. The CACNA1A mRNA encodes two proteins, the ?1A subunit of the neuronal P/Q-type voltage-gated Ca2+ channel, and ?1ACT, a protein discovered by my group, that also bears the polyQ tract. This is critical, since homozygous CACNA1A deletion is lethal, suggesting that complete gene silencing approach to treatment will be harmful. However, studies using both mouse genetic models and human-derived neuronal cells have convincingly excluded a role of the expanded polyQ tracts in altering P/Q channel function in SCA6. Furthermore, our recent evidence suggests that SCA6 is mediated by the second CACNA1A gene product, ?1ACT, a transcription factor (TF) critical for cerebellar cortical development, dispensable in mice in adulthood. ?1ACT with expanded polyQ tract (?1ACTSCA6) impairs its molecular function and is toxic in Purkinje cells (PCs). There are two goals to this project: 1. To determine whether expanded ?1ACT causes SCA6 by a disruption of its TF function or by a direct toxic effect. 2. Prove that selective silencing of ?1ACT, the second CACNA1A gene product, is an effective treatment for SCA6. ?1ACT is expressed by a novel mechanism via an internal ribosomal entry site (IRES) within CACNA1A, molecular treatments, targeted to the IRES to block ?1ACT translation and can prevent disease in an acute SCA6 model. The long range goal of this project is develop a treatment for SCA6. To address this, as defined in the Aims below, we will identify the key protein domains and post-translational modifications that determine the physiological and pathological properties of ?1ACT and determine the effects of ?1ACTWT and ?1ACTSCA6 on chromatin accessibility and dsDNA breaks. We will demonstrate that IRES-directed therapies are an effective treatment for SCA6. Aim 1. Define the key protein domains and post-translational modifications that determine the transcription factor properties of ?1ACT and its toxicity in SCA6. Aim 2. Determine the effects of ?1ACTWT and ?1ACTSCA6 on chromatin accessibility and dsDNA Aim 3. Prove that IRES-directed therapies correct the molecular, cellular and clinicopathological features of SCA6.